The present invention relates to an insulated gate field effect transistor and a method of manufacturing the same, and an image pickup device and a method of manufacturing the same, and particularly to an insulated gate field effect transistor and a method of manufacturing the same, and an image pickup device and a method of manufacturing the same that prevent occurrence of punch-through and injection.
Conventionally, MOS and MONOS transistors forming an output circuit of a CCD image pickup element use a depleted back gate to obtain high gain.
A conventionally known drive transistor forming an output circuit of a CCD (charge-coupled type) image pickup element will be described with reference to a schematic structure sectional view of FIG. 9. As shown in FIG. 9, a gate electrode 32 is formed on an N type semiconductor substrate 11 with a gate insulating film 31 interposed between the N type semiconductor substrate 11 and the gate electrode 32. A source region 33 and a drain region 34 of a P type are formed in the semiconductor substrate 11 on both sides of the gate electrode 32. In addition, a P+ type diffusion layer 22 is formed in the semiconductor substrate 11 at a position deeper than the source region 33 and the drain region 34. The drive transistor 300 is thus formed.
Potential under a channel of the drive transistor 300 will be described with reference to a potential diagram of FIG. 10. As shown in FIG. 10, the drive transistor 300 has a depleted back gate to obtain high gain as a source follower type output circuit.
A conventionally known constant-current transistor of an output circuit of a CCD image pickup device will next be described with reference to a schematic structure sectional view of FIG. 11. As shown in FIG. 11, a gate electrode 42 is formed on an N type semiconductor substrate 11 with a gate insulating film 41 interposed between the N type semiconductor substrate 11 and the gate electrode 42. A source region 43 and a drain region 44 of a P type are formed in the semiconductor substrate 11 on both sides of the gate electrode 42. An N+ type diffusion layer 45 is formed between the source region 43 and the drain region 44 in the semiconductor substrate 11 with a region of the semiconductor substrate 11 left on the gate electrode 42 side. Further, a P+ type diffusion layer 22 is formed in the semiconductor substrate 11 at a position deeper than the source region 43 and the drain region 44. The constant-current transistor 400 is thus formed.
Potential under a channel of the constant-current transistor 400 will be described with reference to a potential diagram of FIG. 12. As shown in FIG. 12, a back gate of the constant-current transistor 400 is in a so-called neutral state (a zero potential state). In addition to the constant-current transistor 400 having the neutral back gate because no gain is required, there is a constant-current transistor 400 with a depleted back gate. A comparison between the potential of the drive transistor 300 shown in FIG. 10 and the potential of the constant-current transistor 400 shown in FIG. 12 indicates that channel potential differs in order to obtain an optimum operating point in consideration of gain and frequency characteristics and that coupling capacitance differs.
Further, the drive transistor and the constant-current transistor differ from each other in coupling capacitance between the substrate and the channel. A potential diagram of FIG. 13 showing potential under the channel of the drive transistor 300 at the time of application of an electronic shutter and a potential diagram of FIG. 14 showing potential under the channel of the constant-current transistor 400 at the time of application of the electronic shutter show that there appears a difference in back gate effect on the channel. When the electronic shutter is applied in this state, the operating point of the output circuit is varied, and thus a “shutter step,” which refers to a step-like level difference appearing in quality of a picked-up image, occurs.
In addition, insulated gate field effect transistors such as MOS transistors, MONOS transistors and the like included in a CCD image pickup device are diversified, so that there are various forms of operating point and channel potential. For example, there is a transistor requiring high gain characteristics for a purpose, such for example as a reset gate having a similar structure as that of FIG. 11. FIG. 15 is a potential diagram showing potential under a channel of such a conventional reset gate transistor at the time of application of an electronic shutter. In the drawing, a potential curve represented by a broken line is a potential curve before the application of the electronic shutter, and a potential curve represented by a solid line is a potential curve after the application of the electronic shutter. After the application of the electronic shutter, potential difference is reduced, and therefore punch-through from the channel to the substrate or injection from the substrate to the channel tends to occur.
However, the source follower drive transistor and the constant-current transistor in the output circuit of an image pickup device differ from each other in potential of the back gate in order to obtain an optimum operating point in consideration of gain and frequency characteristics. Therefore, when the electronic shutter is applied, a phenomenon occurs in which output level is varied due to difference in the back gate effect between the drive transistor and the constant-current transistor (hereinafter referred to as a “shutter step”), and a problem of step-like output difference appearing in quality of a picked-up image tends to occur.
Moreover, the channel potential of the constant-current transistor has recently been increased for still higher gain, so that punch-through from the channel to the substrate tends to occur. Furthermore, various circuits such as a bias circuit and the like have been included in a CCD image pickup device. Depending on a purpose, some reset gates and transistors forming these circuits have a depleted back gate for high gain. These transistors also tend to cause punch-through and injection from the substrate.
It is an object of the present invention to provide an insulated gate field effect transistor, an image pickup device using the same, and manufacturing methods thereof that suppress occurrence of a shutter step and suppress occurrence of punch-through and injection.